Composite anode material including surface-stabilized active material particles and methods of making same

ABSTRACT

Composite anode materials and methods of making same, the anode materials including capsules including graphene, reduced graphene oxide, graphene oxide, or a combination thereof, and particles of an active material disposed inside of the capsules. The particles may each include a core and a buffer layer surrounding the core. The core may include crystalline silicon, and the buffer layer may include a silicon oxide, a lithium silicate, carbon, or a combination thereof.

TECHNICAL FIELD

This invention relates to the art of electrochemical cells, and moreparticularly, to composite anode materials including surface-stabilizedactive material particles encapsulated in graphene or reduced grapheneoxide, and methods of making the same.

BACKGROUND

Lithium (Li) ion electrochemical cells typically have a relatively highenergy density and are commonly used in a variety of applications whichinclude consumer electronics, wearable computing devices, militarymobile equipment, satellite communication, spacecraft devices andelectric vehicles. Lithium ion cells are particularly popular for use inlarge-scale energy applications such as low-emission electric vehicles,renewable power plants, and stationary electric grids. Additionally,lithium-ion cells are at the forefront of new generation wireless andportable communication applications. One or more lithium ion cells maybe used to configure a battery that serves as the power source for theseapplications. The explosion in the number of higher energy demandingapplications and the limitations of existing lithium-ion technology areaccelerating research for higher energy density, higher power density,higher-rate charge-discharge capability, and longer cycle life lithiumion cells.

Lithium ion cells are mainly composed of an anode, for example,graphite, a carbonate-based organic electrolyte, and a cathodecomprising a cathode active material, for example, lithium cobalt oxide(LiCoO₂). Lithium ions are intercalated and deintercalated between theanode and the cathode through the electrolyte during discharge andcharge. When electrical energy is removed from the cell to supply power,or is discharging, lithium ions move from the negative electrode (anode)to the positive electrode (cathode). When the cell is suppliedelectrical energy for conversion to stored chemical energy or ischarging, the opposite occurs. Lithium ions generally move from thepositive electrode (the cathode) to the negative electrode (the anode)during charging. For example, the theoretical capacities of a graphiteanode and a LiCoO₂ cathode are 372 mAh/g and less than 160 mAh/g,respectively. These theoretical charge capacities, however, are oftentoo low for the recent surge in higher energy demanding applications.

Incorporating silicon within a carbon based anode significantlyincreases the capacity of the anode material. Silicon has a theoreticalcapacity of about 4,200 mAh/g, which significantly increases cellcapacity when incorporated within an electrode comprising graphite,graphene, or other carbon based active material. Examples of electrodescomprising graphene and silicon are provided in U.S. Pat. No. 8,551,650to Kung et al. and U.S. patent application publication number2013/0344392 to Huang et al., both of which are incorporated fullyherein by reference.

Furthermore, it is generally understood that silicon incorporated withinthese electrodes typically undergoes a significant volume expansion ofup to 400 percent upon the insertion and extraction of lithium duringthe cycling process. As a result of this significant volume increase,the silicon within the electrode structure experiences a significantmechanical stress which typically causes the material to fracture andimpart defects within its structure. Such structural degradation of thesilicon within the active material typically leads to a reduction inintercalation and de-intercalation of the lithium ions within the activematerial which causes a reduction in capacity and/or cycle life. Inaddition, the mechanical degradation of the silicon typically results inthe electrical disconnection of the silicon within the active material.This electrical disconnection of the silicon caused by mechanicaldegradation of the silicon particles generally leads to a furtherreduction of cycle life and increased capacity loss.

Accordingly, there is a need for a lithium cell with increased capacityand cycle life. The present application, therefore, addresses thisproblem by disclosing an electrochemically active material for use in alithium ion that increases cycle life.

SUMMARY

Various embodiments provide a composite anode material comprisingcapsules comprising graphene, reduced graphene oxide, graphene oxide, ora combination thereof, and active material particles disposed inside ofthe capsules. Each particle may include a core and a buffer layersurrounding the core, with the buffer layer and the core comprisingdifferent materials.

Various embodiments provide an anode comprising: capsules comprisinggraphene, reduced graphene oxide, graphene oxide, or a combinationthereof; active material particles disposed inside of the capsules; anda binder. Each particle may include a core and a buffer layersurrounding the core, with the buffer layer and the core comprisingdifferent materials.

Various embodiments provide a method of forming an anode activematerial, the method comprising: heating crystalline silicon particlesin an inert atmosphere at a first temperature ranging from about 700° C.to about 900° C.; heating the particles at the first temperature in anoxidizing atmosphere, for a first time period, to form oxidizedparticles comprising a silicon oxide coating surrounding a crystallinesilicon core; and cooling the oxidized particles in an inert atmosphere.

In various embodiments, the method may further include: mixing theoxidized particles, a lithium salt, and a solvent to form a mixture;drying the mixture; and heating the dried mixture at a temperatureranging from about 600° C. to about 700° C. in an inert atmosphere for asecond time period, to form active material particles that each comprisea buffer layer comprising a lithium silicate surrounding a crystallinesilicon core.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of an embodiment of astructure of a particle of a composite anode material, according tovarious embodiments of the present disclosure.

FIG. 2 illustrates a cross-sectional view of an embodiment of acore-shell structure of an active material particle, according tovarious embodiments of the present disclosure.

FIG. 3A shows an embodiment of a flow chart of the first active materialparticle modification process that may be used to modify the structureof the active material particle.

FIG. 3B shows an embodiment of a flow chart of the second activematerial particle modification process that may be used to modify thestructure of the active material particle.

FIG. 4 is an x-ray diffraction pattern of particles of an embodiment ofthe active material particle of the present application.

FIG. 5 is a graph that shows capacity retention percent as a function ofcharge/discharge cycle of lithium ion test cells having an electrodecomprising the electrochemically active material of the presentapplication in comparison to control cells constructed with electrodescomprising graphene encapsulated silicon particles.

FIG. 6 is a graph that shows capacity retention percent as a function ofcharge/discharge cycle of lithium ion test cells constructed with anelectrode comprising the electrochemically active material of thepresent application in comparison to control cells constructed withelectrodes comprising graphene encapsulated silicon.

DETAILED DESCRIPTION

In general, the present invention relates to the structure and method offormation thereof of an electrochemically active material comprisingsilicon and graphene. The active material may be formed into anelectrode that is incorporatable within an electrochemical cell. Morespecifically, the present invention relates to the structure and methodof formation thereof of an electrochemically active material comprisingsilicon and graphene that may be formed into an electrode for use with alithium ion electrochemical cell that is designed to provide increasedcapacity and capacity retention.

In various embodiments, the electrochemically active material of thepresent application comprises at least one electrochemically activecomponent that is encased or encapsulated within a capsule or shellcomposed of at least one of graphene, partially reduced graphene oxide,or graphene oxide. In various embodiments, the active material particlecomprises a core-shell structure. In various embodiments, the core-shellstructure of the active material particle comprises a core material thatis encased or encapsulated within a buffer layer. The buffer layercomprising a material that is different than the core material.

In various embodiments the buffer layer that surrounds the corematerial, such as silicon or a silicon-based material, allows formechanical expansion of the core material so that the active materialparticles are less likely to fracture or mechanically degrade due toexpansion. Thus, the capacity of the resulting lithium-ion cell thatcomprises a composite anode material of the present application ismaintained over multiple charge-discharge cycles.

The particles of the electrochemically active material of the presentapplication may comprise a multitude of structures, including but notlimited to, a crumpled, paper ball-like structure, a core-shellstructure, or a substantially spherical-like shape. In addition, theparticle structure of the electrochemically active material of thepresent application provides a compact structure that increaseselectrical conductivity and decreases the distance in which lithium ionsdiffuse. Furthermore, the particle structure of the electrochemicallyactive material provides for an internal void space 20 within thestructure of each of the particles that tolerates swelling of the corematerial, such as silicon or a silicon-based material, and minimizesknown negative effects that compromise achievable capacity, thuspreserving capacity as the cell is charged and discharged. In variousembodiments, graphene, partially reduced graphene oxide, graphene oxide,or a combination thereof, forms a capsule having at least one internalvoid space 20 therein devoid of graphene, which encases or encapsulatesat least one active material particle therein. The present applicationfurther discloses a method of fabricating the novel electrochemicallyactive material, and a method of forming the electrochemically activematerial of the present application into an electrode for incorporationwithin an electrochemical cell. In various embodiments, the electrode ofthe present application is an anode or negative electrode that may beincorporated within a secondary lithium-ion electrochemical cell.

As defined herein a “secondary” electrochemical cell is anelectrochemical cell or battery that is rechargeable. “Capacity” isdefined herein as the maximum amount of energy, in ampere-hours (Ah),that can be extracted from a battery under certain specified conditions;the amount of electric charge that can be delivered at a rated voltage.Capacity may also be defined by the equation: capacity=energy/voltage orcurrent (A)×time (h). “Energy” is mathematically defined by theequation: energy=capacity (Ah)×voltage (V). “Specific capacity” isdefined herein as the amount of electric charge that can be deliveredfor a specified amount of time per unit of mass or unit of volume ofelectrode active material. Specific capacity may be measured ingravimetric units, for example, (A·h)/g or volumetric units, forexample, (A·h)/cc. Specific capacity is defined by the mathematicalequation: specific capacity (Ah/Kg)=capacity (Ah)/mass (Kg). “Ratecapability” is the ability of an electrochemical cell to receive ordeliver an amount of capacity or energy within a specified time period.Alternately, “rate capability” is the maximum continuous or pulsedoutput current a battery can provide per unit of time. Thus, anincreased rate of charge delivery occurs when a cell discharges anincreased amount of current per unit of time in comparison to asimilarly built cell, but of a different anode and/or cathode chemistry.“C-rate” is defined herein as a measure of the rate at which a batteryis discharged relative to its maximum capacity. For example, a 1C ratemeans that the discharge current will discharge the entire battery in 1hour. “Power” is defined as time rate of energy transfer, measured inWatts (W). Power is the product of the voltage (V) across a battery orcell and the current (A) through the battery or cell. “C-Rate” ismathematically defined as C-Rate (inverse hours)=current (A)/capacity(Ah) or C-Rate (inverse hours)=1/discharge time (h). Power is defined bythe mathematical equations: power (W)=energy (Wh)/time (h) or power(W)=current (A)×voltage (V).

A “composite anode material” may be defined as a material that may beconfigured for use as an anode in an electrochemical cell, such as alithium ion rechargeable battery and that includes surface stabilizedactive material particles encapsulated in a capsule of graphene, reducedgraphene oxide, or graphene oxide. The anode material may include anodematerial particles, a binder, and may optionally include a conductivityenhancing agent. An “electrochemically active material” or “activematerial” is defined herein as a material that inserts and releasesions, such as ions in an electrolyte, to store and release an electricalpotential. A “capsule” or “shell” is defined herein as a structure thatsurrounds and encases an active material particle. An “active materialparticle” is defined herein as a particle of the active material thatmay be disposed within the capsule, and includes a core surrounded byone or more buffer layers. In some embodiments, the core material may bean electrochemically active material. A “buffer layer” is a layer ofmaterial that surrounds and encases the core of an active materialparticle. “Void space” is defined herein a space between the activematerial particles and an interior surface of the capsule or shellstructure.

FIG. 1 illustrates a composite anode material particle 10 included in acomposite anode material, according to various embodiments of thepresent disclosure. Referring to FIG. 1, while one particle 10 is shown,the composite anode material may include a plurality of the anodematerial particles 10, l according to various embodiments of the presentdisclosure. In addition, the anode material may also include a binderand optionally a conductivity enhancing agent. The anode materialparticle 10 may comprise a shell or capsule 12 that encloses and/orencapsulates at least one active material particle 14 therein.

In various embodiments, the capsule 12 may include graphene, reducedgraphene oxide, or graphene oxide. Herein, reduced graphene oxide may beformed by partially, or substantially completely, reducing grapheneoxide. The capsule 12 may have a crumpled structure. The capsule 12 maybe substantially spherical or ovoid. In some embodiments, the capsule 12may have a smooth or roughened outer surface morphology. The capsule 12provides a void space 20 therein that allows for controlled expansion ofthe active material particles 14 enclosed therein.

FIG. 2 illustrates an embodiment of an active material particle 14. Asshown in the embodiment, each active material particle 14 comprises acore 16, and a buffer layer 18 surrounding the core 16. In variousembodiments, the core 16 may comprise an electrochemically activematerial such as metallic silicon (Si). The buffer layer 18 may comprisea silicon oxide, a lithium silicate, carbon-based materials, or acombination thereof. In various embodiments, the buffer layer 18 maycomprise one or more silicon oxides (SiO_(x)), wherein 0<x<0.8, one ormore lithium silicates, such as Li₂Si₂O₅, Li₂SiO₃, and Li₄SiO₄, or anycombination thereof. For example, the buffer layer 18 may comprise acomposite matrix of oxides, silicates, and/or carbon phases and/orlayers. Such a composite buffer layer may provide for unexpectedlyimproved cycle life when incorporated into a battery.

The buffer layer 18 provides for a mechanical buffer that surrounds eachcore 16. More specifically, the buffer layer 18 provides for amechanical support that minimizes mechanical degradation and spalling ofthe core 16. Thus the buffer layer 18 helps to minimize fracture of thecore 16, due to mechanical expansion. For example, the buffer layer 18may allow the core 16 to be formed of crystalline silicon, which wouldotherwise suffer from mechanical degradation.

In various embodiments, the core 16 may have an average particle sizethat is less than about 500 nm, such as from about 50 nm to about 400nm. The buffer layer 18 may have a thickness that ranges from about 1 nmto about 50 nm, such as from about 1 nm to about 25 nm, from about 5 nmto about 15 nm, or from about 7 nm to about 12 nm. In variousembodiments, the thickness of the buffer layer 18 may be about 10 nm orless.

In various embodiments, the anode material 10 may comprise about 70weight percent graphene, reduced graphene oxide, or graphene oxide, andabout 30 weight percent active material particles 14. In variousembodiments, the anode material particles 10 (e.g., the capsules 12) mayhave an average particle size that ranges from about 0.5 μm to about 10μm. In various embodiments, the average particle size of the anodematerial particles 10 may range from about 2 μm to 5 μm.

In various embodiments, the average particle size of the active materialparticles 14 may range from about 30 nm to about 100 nm. Furtherembodiments of anode materials and structures thereof are disclosed inU.S. Patent Application Publication Numbers 2013/0004798 and2013/0344392, both to Huang et al., all of which are incorporated fullyherein by reference.

In various embodiments, the active material particles 14 may befabricated utilizing a two-step active material modification process.FIGS. 3A and 3B illustrate flow charts of the first and second processesthat comprise the two-stage active material modification process. Invarious embodiments, the first process of FIG. 3A comprises heating corematerial particles, such as crystalline silicon particles, in anenriched oxygen atmosphere, to form an oxide buffer layer, such as asilicon oxide layer, on the surfaces of core material particles.

In various embodiments, silicon particles are first heated in an inertgas environment at a temperature that ranges from about 700° C. to about1,000° C., such as a temperature ranging from about 700° C. to about900° C., or from about 750° C. to about 850° C. The inert gas maycomprise argon, helium, neon, xenon or combinations thereof. In variousembodiments, the silicon particles may have an average particle sizethat ranges from about 25 nm to about 5 μm, such as from about 50 nm toabout 1 μm, or from about 50 nm to about 500 nm.

Once the silicon particles are heated to the desired temperature, thesilicon particles are exposed to an oxidizing gas, such as compressedair or an oxygen enriched gas. In various embodiments, the siliconparticles are heat treated in the oxidizing gas for a time periodranging from about 15 minutes to about an hour. The oxidizing gas may becompressed to form an enriched oxygen gas comprising about 10 to about40 percent oxygen, such as from about 15 to about 25 percent oxygen.Alternatively, the oxidizing gas may be pure oxygen. It is noted thatlarger size silicon particles may require longer dwell times atincreased temperatures in the enriched oxygen environment to form abuffer layer on the core particles at a desired thickness andcomposition.

In various embodiments, silicon particles having a particle size ofabout 50 nm may comprise a buffer layer 18 having a thickness thatranges from about 4 nm to about 10 nm. For larger size silicon, such ascore particles having an average particle size of about 400 nm, thethickness of the buffer layer may be greater. For example, a bufferlayer, formed on core particles having an average particle size of about400 nm, may have a thickness ranging from about 10 nm to about 50 nm.

For example, crystalline silicon core particles having an averageparticle size of about 50 nm may be exposed to a compressed airenvironment at a temperature of about 750° C., for about 15 minutes,whereas core particles having a particle size of about 400 nm may beexposed to a compressed air environment at about 800° C., for about 30minutes. Once the buffer layers have been formed, the resultant buffered(e.g., oxidized) active material particles may be incorporated into ananode material for use as an electrode of a lithium ion cell.

In some embodiments, the buffered active material particles may befurther processed in the second process shown in FIG. 3B. The secondprocess further modifies the composition of the buffer layers, such thatthe buffer layers comprise one or more lithium silicates.

For example, the second process may comprise mixing the buffered activematerial particles, a lithium salt, and a solvent to form a precursorsolution, which may then be mixed for a time period ranging from about 1to about 2 hours. For example, the mixing may be performed bysonication.

The solvent may include 3-isopropenyl-6-oxoheptanol (IPOH), for example.The lithium salt may include any suitable lithium salt, such as, lithiumacetate dehydrate, LiO₂, LiS, LiPF₆, bis(oxalato)borate (LiBOB),oxalyldifluoroborate (LiODFB) and fluoroalkylphosphate (LiFAP), or thelike. When a lithium salt including an organic component, such aslithium acetate dehydrate, is used, the resulting buffer layer mayinclude carbon or a carbon layer.

In some embodiments, the buffer layer includes silicon oxide and theprecursor solution includes oxidized active material particles at asilicon oxide to lithium salt weight ratio that ranges from 1:0.5 to1:0.75, such as from about 1:0.6 to 1:0.7, or about 1:0.67. In variousembodiments, the weight ratio of the buffer layer to lithium salt isappropriately selected such that the lithium salt reacts with the bufferlayer without reacting with the core particle.

In various embodiments, the precursor solution includes an amount ofsolvent sufficient to create a solids content that ranges from about 5to about 15 wt %, with respect to the total weight of the precursorsolution. In various embodiments, the IPOH is added to create a solidscontent of about 10 wt %. After the IPOH is added to the desired solidscontent, the mixture is mixed for about 1 to about 2 hours. The mixturemay be sonicated to ensure a homogenous mixture.

After mixing is complete, the solution is dried to remove the solvent.In various embodiments, the resultant particles are then heat treated inan inert atmosphere, for example, argon, at a temperature of about 650°C., for from about 10 minutes to about 1 hour, to form active materialparticles having a core comprising silicon and a buffer layer comprisinga lithium silicate, such as Li₂Si₂O₅, Li₂SiO₃, Li₄SiO₄, or combinationsthereof.

FIG. 4 is an x-ray diffraction (XRD) pattern of the active materialparticles after the step two active material particle modificationprocess. As illustrated in FIG. 4, the XRD pattern shows that theparticles comprise a combination of Si, Li₂SiO₃ and Li₂Si₂O₅. In variousembodiments, the composition of the buffer layer may comprise a mixtureof Li₂Si₂O₅ and Li₂SiO₃ having a weight ratio of 15:9 respectively.

In addition to crystalline silicon, it is further contemplated that thecore material of the active material particles may comprise othermaterials that include, but not limited to, silicon oxide, titaniumoxide, graphite, carbon, metal nanoparticles (e.g., silver or platinum),salts, such as CsCl, tin (Sn), tin oxide, antimony (Sb), aluminum (Al),germanium (Ge), gallium (Ga), magnesium (Mg), zinc (Zn), lead (Pb),bismuth (Bi), lithium titanium oxide, their alloys, intermetallics,other monometallic, bimetallic, or multi metallic materials, or oxidicor sulfide materials, and mixtures thereof, in a nano-particle form.Some specific examples may include metal oxides, such as ZnO, Co₃O₄,Fe₂O₃, MnO₂, Mn₃O₄, MnO, Fe₃O₄, NiO, MoO₂, MoO₃, CuO, Cu₂O, CeO₂, andRuO₂. In various embodiments, particles of the active materialparticles, such as those formulated by either or both of the first andsecond processes, are then incorporated within the anode material of thepresent application.

In various embodiments, the anode material may be formed byencapsulating the active material particles within a capsule of at leastone of graphene, reduced graphene oxide, or graphene oxide. In variousembodiments, a spray drying process in which heat from the spray dryingprocess crumples the graphene, reduced graphene oxide, or graphene oxidearound the active material particles.

In various embodiments, the shape of the particles of the anode materialparticles may be customized by adjusting the parameters of thespray-drying process. For example, the anode material particles may havea structure that is specifically engineered to be of a substantiallycrumpled, paper ball-like structure, a core-shell structure, or asubstantially spherical-like shape in which multiple sheets of graphene,partially reduced graphene oxide, or graphene oxide come together toform the capsule or shell of the particle structure.

Referring again to FIGS. 1 and 2, the buffer layer 18 may be designed tominimize the mechanical swelling of the core 16. In various embodiments,the buffer layer 18 acts as a buffer that allows for core expansion, andthus, minimizes the possibility of fracture of the core 16. In addition,the structure of the capsule 12 may be designed to tolerate further coreparticle swelling therein. Thus, the capsule 12 may be designed tofurther minimize capacity loss. The capsule 12 provides a mechanicallyrobust design that is capable of expanding and contracting in concertwith the swelling of the modified core particles therein. This swellingtolerance of the capsule 12 helps preserve the capacity and minimizecapacity loss of the resulting electrode and electrochemical cell.

In various embodiments, the capsule 12 which encases the active materialparticles 14, increases the electrical conductivity between the capsule12 and core 16 as a result of the increased contact of the activematerial particles 14 with an interior surface of the capsule 12. Inaddition, the capsule 12 further enhances the electrical conductivitybetween the core particles 16, a current collector, a supportingsubstrate, and the surrounding area of the particles.

In various embodiments, the electrode of the present application may beconstructed from an electrode slurry that comprises an anode material ofthe present application, a binder, a conductive additive, and a solvent.Appropriate proportions of the components of the anode material andother constituents are first mixed to form the electrode slurry. Oncefabricated, the electrode slurry is applied to a surface of an electrodecurrent collector (not shown), composed of an electrically conductivematerial, such as copper, to create an electrode for use in anelectrochemical cell. After the electrode slurry has been applied to thesurface of a substrate, such as a current collector (not shown), theelectrode slurry is dried and calendared to compress the electrode to adesired porosity.

A dispersant (including surfactants, emulsifiers, and wetting aids), athickening agent (including clays), defoamers and antifoamers, biocides,additional fillers, flow enhancers, stabilizers, cross-linking andcuring agents may be added to the slurry mixture to ensure a homogenousmixture thereof. Examples of dispersants include, but are not limitedto, glycol ethers (such as poly(ethylene oxide), block copolymersderived from ethylene oxide and propylene oxide (such as those soldunder the trade name Pluronic® by BASF), acetylenic diols (such as2,5,8,11-tetramethyl-6-dodecyn-5,8-diol ethoxylate and others sold byAir Products under the trade names Surfynol® and Dynol®), salts ofcarboxylic acids (including alkali metal and ammonium salts), andpolysiloxanes. Additional examples of dispersants may include sodiumdodecanoate, alkanolamide, lanolin, polyvinylpyrrolidone, sodium alkylsulfate, sodium alkyl sulfonate, lecithin, polyacrylate, sodiumsilicate, and polyethoxy, nitrocellulose and Triton® X-100 a dispersanthaving the chemical formula, (C₂H₄O)nC₁₄H₂₂O produced by DOW Chemicalcompany of Midland, Mich. Examples of thickening agents includelong-chain carboxylate salts (such aluminum, calcium, zinc, salts ofstearates, oleates, palmitates), aluminosilicates (such as those soldunder the Minex® name by Unimin Specialty Minerals and Aerosil® 9200 byEvonik Degussa), fumed silica, natural and synthetic zeolites. Invarious embodiments, the slurry mixture may comprise from about 0.01 toabout 1.0 weight percent dispersant and/or thickening agent.

In various embodiments, binders may include but are not limited to, afluoro-resin powder such as polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVDF), poly (acrylic) acid,polyethylenetetrafluoroethylene (ETFE), polyamides, and polyimides, andmixtures thereof. Additional binders may include, but are not limitedto, polyethylene (UHMW), styrene-butadiene rubber, cellulose,polyacrylate rubber, and copolymers of acrylic acid or acrylate esterswith polyhydrocarbons such as polyethylene or polypropylene, andmixtures thereof. Solvents may include but are not limited to, deionizedwater, ethanol, isopropyl alcohol, ethylene glycol, ethyl acetate, polarprotiac solvents, polar aprotic solvents, N-methyl-2-pyrrolidone, andcombinations thereof.

Conductive additives may include but are not limited to, carbon black,an electrically conductive polymer, graphite, or a metallic powder suchas powdered nickel, aluminum, titanium and stainless steel.

In various embodiments, the electrode active slurry of the presentapplication may comprise from about 50 to about 85 weight percent of theelectrochemically active material 10, from about 10 to about 25 weightpercent binder, from about 2 to about 7 weight percent the conductiveadditive and the remainder comprising the solvent or solvent solution.In various embodiments, the electrode active slurry may have a solidscontent that ranges from about 15 to about 35 weight percent. In variousembodiments, the slurry may have a solids content that ranges from about20 weight percent to about 30 weight percent. The solids content of theslurry allows for an ideal slurry viscosity that enhances a uniformcoating on a substrate or current collector.

Each of the constituents of the electrode may be added separately, oralternatively, as separate electrode suspensions comprising at leastportions of the electrode active slurry component materials that arecombined to create the electrode slurry of the present application. Invarious embodiments, the components of the electrode active slurry aremixed to a uniform consistency. The slurry components may be mixed usinga variety of unlimited techniques such as ball milling or planetarymixing.

In various embodiments, mixing times may range from about 30 minutes to2 hours depending on batch size to achieve a uniform, homogenous slurrymixture. Milling media may also be added to the slurry to aid increating a homogenous mixture. The electrode slurry may be furtherdispersed through manual or automated agitation. Such agitation mayinclude physical shaking or rocking of the suspension. In addition, theelectrode slurry may be subjected to ultrasonication for about 30seconds to about 30 minutes to further disperse the silicon and carbonparticles and help to create a homogeneous electrode suspension mixture.The electrode slurry should be prepared such that it is able toadequately flow and adhere onto the surface of the substrate. In variousembodiments, the electrode slurry may have a viscosity ranging fromabout 0.1 Pa·S to about 1,000 Pa·S at a shear rate of between about 0.1to 1,000 s⁻¹.

After the electrode slurry has been formulated, the slurry is applied tothe surface of a substrate. In various embodiments, the electrode slurrymay be applied to the surface of a substrate comprising a metal, apolymer, a ceramic, and combinations thereof. Non-limiting examples ofsubstrate materials may include but are not limited to, metals such ascopper, aluminum, nickel, and their alloys, polymers such aspolyethylene, polyimide, and polyether ether ketone (PEEK), as well asalumina and various glasses. In various embodiments, the electrodeslurry is applied to the surface of a current collector such as thosecomposed of copper, nickel, aluminum, and combinations thereof.

In various embodiments, the electrode slurry may be applied to a desiredthickness ranging from a few nanometers to a few micrometers using avariety of non-limiting application techniques. In various embodiments,the thickness of the applied electrode slurry may range from about 5 μmto about 50 μm. These application techniques may include but are notlimited to, the use of Meyer rod coating, the use of a doctor blade orknife, spray coating, dip coating, spin coating or brush application. Inaddition, the electrode slurry layer may be applied to a substratesurface through the use of thick-film or thin-film processingtechniques.

Furthermore, in various embodiments, the surface of the substrate may bemodified prior to the application of the electrode slurry to improveadhesion to the substrate surface. Examples of such substrate surfacemodifications include, but are not limited to, surface etching orsurface roughening through the use corona treatment, acid etching, sandblasting or bead blasting.

After the electrode slurry has been applied to the surface of thesubstrate, it is then dried to remove at least a majority of thesolvent. In various embodiments, the electrode slurry layer may be driedusing convection air drying, a UV light source and/or an infrared lightsource. Additionally, the electrode slurry may be dried through the useof freeze drying, vacuum drying, or through osmosis.

In addition, the slurry may be dried through application of a heatsource that is applied directly to the exposed surface of the electrodeslurry coating or alternatively, the electrode slurry may also be driedthrough application of a heat source to the underlying substrate, orboth. Furthermore, the electrode slurry may be dried in a variety ofnon-limiting atmosphere conditions having a variety of atmosphericpressures. For example, a gas such as nitrogen may be used to controlthe drying process. In various embodiments, the applied electrode slurryis dried under a UV-light source for about 1 hour afterward it is thendried in an oven at about 80° C. for between about 2 to 24 hours, oruntil the solvent has been substantially removed from the resultingelectrode structure.

In various embodiments, the electrode slurry is dried to a thicknessranging from about 5 μm to about 50 μm. In various embodiments, theelectrode slurry is dried to a thickness ranging from about 8 μm toabout 15 μm. In various embodiments, the thickness of the driedelectrode layer(s) is targeted to achieve an increase in electricalpower. The reduced electrode thickness minimizes the diffusion distanceand which enables rapid lithium ion migration within the electrodestructure.

The drying process of the present application allows for theelectrochemically active material 10, to maintain the internal voidspace 20 within the capsule particle structure. Subjecting the driedelectrode to further elevated heating conditions, such as sintering, maylead to a decrease in electrical conductivity of the material and, inaddition, may cause the silicon or silicon oxide within the particle ofthe electrochemically active material 10 of the present application tobecome fused to the graphene or graphene oxide capsule 12. As a result,the capacity generated by the particles may decrease.

After the slurry is dried, the formed electrode may be calendered. Invarious embodiments, the calendaring process compresses the electrodethus decreasing the void space within the dried electrode. In variousembodiments, the dried electrode may be calendered to achieve a targetvoid space and internal porosity that provides for increased lithiumdiffusion, in addition to structural integrity. In various embodiments,the calendaring process may utilize a roller, or another such tool, thatis rolled over the dried electrode to ensure a proper internal porosity.In various embodiments, the calendaring process may range from about 30seconds to about 5 minutes depending upon the thickness of the electrodeand the desired internal porosity. In various embodiments, the electrodeinternal porosity may range from about 40 percent to about 60 percent.In various embodiments, the internal porosity may be about 50 percent.Internal porosity is measured by the following equation:

${{Porosity}\mspace{14mu} (\%)} = {1 - \left( \frac{{measured}\mspace{14mu} {density}}{{theoretical}\mspace{14mu} {density}} \right)}$

where the measured density is measured by dividing the mass of the driedelectrode by its volume and the theoretical density is the density ofthe electrode active material that is 100 percent dense. The theoreticaldensity is assumed to be 2.25 g/cubic centimeter. In variousembodiments, constructing the electrode to a targeted optimal internalporosity provides additional channels within which lithium ions maydiffuse while also providing the required structural integrity for longlife in an electrochemical environment within the cell.

The increased internal porosity thus provides for an increased volume oflithium ions to diffuse through the electrode. In addition, increasingthe internal porosity shortens the distance with which lithium ionstravel through the electrode. As a result of the increased internalporosity, the charge/discharge rate capability of the electrode andresulting electrochemical cell increases.

The electrode thus comprises the composite anode material, and anon-active material comprising a carbon material, which may includeamorphous carbon. In various embodiments, after the drying process, theanode material comprises the active material particles and the capsulesat a weight ratio ranging from about 80:20 to about 6:40, such as about70:30. The electrode may include from about 0.01 weight percent to about5 weight percent of the non-active material.

After the electrode layer(s) are dried and calendered, the electrodelayer(s) and current collector substrate subassembly may be cut to forman electrode of an appropriate shape for incorporation into anelectrochemical cell. Alternatively, the electrode layer may be removedfrom the substrate to form a free-standing electrode. The term ‘freestanding’ is defined herein as sufficiently isolated from itsenvironment, in this case, sufficiently absent the substrate.

In various embodiments, the formulated electrode is an anode or negativeelectrode that is utilized within a secondary lithium-ionelectrochemical cell. The electrochemical cell of the presentapplication further comprises a cathode composed of an electricallyconductive material that serves as the other, positive electrode of thecell. In various embodiments, the cathode is composed of solid materialsand the electrochemical reaction at the cathode involves the conversionof lithium ions that migrate back and forth between the anode, i.e., afirst electrode, and the cathode, i.e., a second electrode, into atomicor molecular forms.

During discharge of the cell, lithium ions flow from the anode ornegative electrode to the cathode or positive electrode. To rechargesuch secondary cells, lithium ions from the cathode or positiveelectrode are intercalated into the anode by applying an externallygenerated electrical potential to the cell. The applied rechargingpotential serves to draw lithium ions from the cathode material, throughthe electrolyte, and into the anode. The solid cathode may comprise acathode active material of a metal oxide, a lithiated metal oxide, ametal fluoride, a lithiated metal fluoride or combinations thereof asdisclosed in U.S. patent application Ser. No. 14/745,747 to Hayner etal., which is assigned to the assignee of the present application andincorporated fully herein by reference. In various embodiments, thecathode active material comprises LiNi_(x)Co_(y)Al_(z)O₂, where x, y andz are greater than 0 and wherein x+y+z=1. Other cathode active materialsmay include but are not limited to lithium cobalt oxide (LiCoO₂),lithium iron phosphate (LiFePO₄) and lithium manganese oxide (LiMn₂O₄).Additional cathode active materials may also include, but are notlimited to, LiNi_(x)Mn_(y)Co_(z)O₂, where 0.3≤x≤1.0, 0≤y≤0.45, and0≤z≤0.4 with x+y+z=1. Furthermore, the cathode active material maycomprise Li_(1+x)Ni_(α)Mn_(β)Co_(γ)O₂, where x ranges from about 0.05 toabout 0.25, α ranges from about 0.1 to about 0.4, β ranges from about0.4 to about 0.65, and γ ranges from about 0.05 to about 0.3.

In a larger scope, the cathode active material may comprise sulfur (S),lithium sulfide (Li₂S), a metal fluoride, a lithium metal fluoride, alithium metal phosphate, and a lithium metal silicate where the metalmay comprise a transition metal from the Periodic Table of Elements,such as iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper(Cu), vanadium (V), chromium (Cr), non-transition metals such as bismuth(Bi), and combinations thereof. Specific examples of cathode activematerials may include MF_(x) where 0≤x≤3, Li_(x)MF_(x) where 0≤x≤3,LiMPO₄, Li₂MSiO₄ composite layered-spinel structures such asLiMn₂O₄-LiMO where M is a transition metal from the Periodic Table ofElements, such as iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni),copper (Cu), vanadium (V), chromium (Cr), a non-transition metal such asbismuth (Bi), and combinations thereof. Lithium rich positive electrodeactive materials of particular interest can also be representedapproximately by the formulaLi_(1+x)Ni_(a)Mn_(b)Co_(c)A_(d)O_(2−Z)F_(Z), where x ranges from about0.01 to about 0.3, a ranges from about 0 to about 0.4, b ranges fromabout 0.2 to about 0.65, c ranges from 0 to about 0.46, d ranges from 0to about 0.15 and Z ranges from 0 to about 0.2 with the proviso thatboth a and c are not zero, and where A is magnesium (Mg), strontium(Sr), barium (Ba), cadmium (Cd), zinc (Zn), aluminum (Al), gallium (Ga),boron (B), zirconium (Zr), titanium (Ti), calcium (Ca), selenium (Ce),yttrium (Y), niobium (Nb), chromium (Cr), iron (Fe), vanadium (V),lithium (Li) or combinations thereof. A person of ordinary skill in theart will recognize that additional ranges of parameter values within theexplicit compositional ranges above contemplated and are within thepresent disclosure.

In various embodiments, the cathode active material is formed by thechemical addition, reaction, or otherwise intimate contact of variousoxides, phosphates, sulfides and/or metal elements, for example, duringthermal treatment, sol-gel formation, chemical vapor deposition, orhydrothermal synthesis in mixed states. The cathode active materialthereby produced may contain metals, oxides, phosphates, and sulfides ofGroups, IB, IIB, IIIB, IVB, VB, VIB, VIIB, VIII, and VIIA which includesthe noble metals and/or other oxide and phosphate compounds. In variousembodiments, a cathode active material is a reaction product ofstoichiometric proportions of at least fully lithiated to non-lithiated,LiNi_(0.8)Co_(0.15)Al_(0.05)O₂.

An exemplary cell of the present application further includes aseparator to provide physical separation between the anode and cathode.The separator is of an electrically insulative material to prevent aninternal electrical short circuit between the electrodes, and theseparator material also is chemically unreactive with the anode andcathode active materials and both chemically unreactive with andinsoluble in the electrolyte. In addition, the separator material has adegree of porosity sufficient to allow flow therethrough of theelectrolyte during the electrochemical reaction of the cell.Illustrative separator materials include non-woven glass, polypropylene,polyethylene, microporous material, glass fiber materials, ceramics,polytetrafluorethylene membrane commercially available under thedesignations ZITEX (Chemplast Inc.), polypropylene membrane,commercially available under the designation CELGARD (Celanese PlasticCompany Inc.) and DEXIGLAS (C. H. Dexter, Div., Dexter Corp.).

The form of the separator typically is a sheet which is placed betweenthe anode and cathode and in a manner preventing physical contacttherebetween. Such is the case when the anode is folded in aserpentine-like structure with a plurality of cathode plates disposedintermediate the anode folds and received in a cell casing or when theelectrode combination is rolled or otherwise formed into a cylindrical“jellyroll” configuration.

The exemplary electrochemical cell of the present application isactivated with a nonaqueous, ionically conductive electrolyteoperatively associated with the anode and the cathode. The electrolyteserves as a medium for migration of lithium ions between the anode andthe cathode during electrochemical reactions of the cell, particularlyduring discharge and re-charge of the cell. The electrolyte is comprisedof an inorganic salt dissolved in a nonaqueous solvent. In variousembodiments, the inorganic salt comprises an alkali metal salt dissolvedin a mixture of low viscosity solvents including organic esters, ethersand dialkyl carbonates and high conductivity solvents including cycliccarbonates, cyclic esters, and cyclic amides.

Additional low viscosity solvents useful with the present applicationinclude dialkyl carbonates such as tetrahydrofuran (THF), methyl acetate(MA), diglyme, trigylme, tetragylme, dimethyl carbonate (DMC),1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), 1-ethoxy,2-methoxyethane (EME), ethyl methyl carbonate, methyl propyl carbonate,ethyl propyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, andmixtures thereof. High permittivity solvents include cyclic carbonates,cyclic esters and cyclic amides such as propylene carbonate (PC),ethylene carbonate (EC), butylene carbonate, acetonitrile, dimethylsulfoxide, dimethyl formamide, dimethyl acetamide, γ-valerolactone,γ-butyrolactone (GBL), N-methyl-2-pyrrolidone (NMP), and mixturesthereof.

In various embodiments, the electrolyte of the present applicationcomprises an inorganic salt having the general formula YAF₆ wherein Y isan alkali metal similar to the alkali metal comprising the anode, and Ais an element selected from the group consisting of phosphorous, arsenicand antimony. Examples of salts yielding AF₆ are: hexafluorophosphate(PF₆), hexafluoroarsenate (AsF₆) and hexafluoroantimonate (SbF₆). Inaddition, other salts may comprise lithium salts including LiPF₆, LiBF₄,LiAsF₆, LiSbF₆, LiClO₄, LiO₂, LiAlCl₄, LiGaCl₄, LiC(SO₂CF₃)₃,LiN(SO₂CF₃)₂, LiSCN, LiO₃SCF₃, LiC₆FSO₃, LiO₂CCF₃, LiSO₆F, LiB(C₆H₅)₄,LiCF₃SO₃, and mixtures thereof. In various embodiments, the electrolytecomprises at least one ion-forming alkali metal salt ofhexafluoroarsenate or hexafluorophosphate dissolved in a suitableorganic solvent wherein the ion-forming alkali metal is similar to thealkali metal comprising the anode. In various embodiments, the alkalimetal salt of the electrolyte comprises either lithiumhexafluoroarsenate or lithium hexafluorophosphate dissolved in a 50/50solvent mixture (by volume) of EC/DMC. In various embodiments, theelectrolyte is 0.8M to 1.5M LiAsF₆ or LiPF₆ dissolved in a 50:50mixture, by volume, of dimethyl carbonate and ethylene carbonate.

In various embodiments, the form of the electrochemical cell is alithium ion cell wherein the anode/cathode couple is inserted into aconductive metal casing. In various embodiments, the casing may comprisestainless steel, although titanium, mild steel, nickel, nickel-platedmild steel, and aluminum are also suitable. The casing may comprise ametallic lid having a sufficient number of openings to accommodate aglass-to-metal seal/terminal pin feedthrough for the cathode and anode.An additional opening may be provided for electrolyte filling. Thecasing header comprises elements having compatibility with the othercomponents of the electrochemical cell and is resistant to corrosion.The cell is thereafter filled with the electrolyte solution describedhereinabove and hermetically sealed, such as by close-welding astainless steel plug over the fill hole, but not limited thereto. Thecell of the present application can also be constructed in acase-positive design.

In various embodiments, the glass-to-metal seal comprises a corrosionresistant glass having from between about 0% to about 50% by weightsilica such as CABAL 12, TA 23 or FUSITE MSG-12, FUSITE A-485, FUSITE425 or FUSITE 435. In various embodiments, the positive terminal pinfeedthrough comprises titanium although molybdenum and aluminum can alsobe used. The cell header comprises elements having compatibility withthe other components of the electrochemical cell and is resistant tocorrosion. The cell is thereafter filled with the electrolyte describedhereinabove and hermetically sealed such as by close-welding a stainlesssteel ball over the fill hole, but not limited thereto. When theionically conductive electrolyte becomes operatively associated with theanode and the cathode of the cell, an electrical potential difference isdeveloped between terminals operatively connected to the anode and thecathode. During discharge, lithium ions migrate from the anode, i.e.,the negative electrode to the cathode, i.e., the positive electrode.During recharge, lithium ions migrate in the opposite direction from thecathode to the anode. In various embodiments, migration of the lithiumions between the anode and cathode occurs in atomic or molecular forms.

Sample lithium-ion cells were constructed with anodes fabricated usingthe material formulation and fabrication methods of the presentapplication. A counter electrode of pure lithium was used to completeeach of the test cells. Lithium ion test cells were constructed with ananode composed of a plurality of electrochemically active materialparticles wherein the electrochemically active material particlesincluded silicon particles encapsulated in graphene and having aparticle size that ranges from about 1 μm to about 10 μm. In variousembodiments, the anode material comprises particles having a crumpledcapsule structure composed of graphene, reduced graphene oxide, grapheneoxide, or a combination thereof. The active material particlescomprising a silicon core with a buffer layer of silicon oxide, lithiumsilicate, or a combination thereof is encapsulated within the graphenecapsule.

In various embodiments, the graphene, partially reduced graphene oxide,or graphene oxide that forms the particle capsule is of a crumpledmorphology. Particles of the active material particles comprising asilicon core 16 and a silicon oxide buffer layer were fabricatedutilizing the first silicon particle fabrication process, as previouslydiscussed. In addition, particles of the active material comprising acrystalline silicon core and a lithium silicate buffer layer were alsofabricated utilizing the first and second silicon particle fabricationprocesses. In various embodiments, the buffer layer comprised eithersilicon oxide (SiO_(x)) where x<0.8, or a lithium silicate comprisingLi₂Si₂O₅, Li₂SiO₃, Li₄SiO₄, or a combination thereof.

Three lithium-ion cells referred to as Test Group 1 were constructedwith an anode comprising an anode material of the present applicationcomprising a crumpled capsule of reduced graphene oxide withencapsulated active material particles having an average particle sizeof about 50 nm that were subjected to step one of the siliconmodification process. Thus, the active material particles within thecrumpled capsule had a core of silicon and a buffer layer of siliconoxide.

In addition, three lithium ion cells referred to as Test Group 2 wereconstructed with an anode comprising an anode material similar to thatof Test Group 1, except that the active material particles had anaverage particle size of about 400 nm.

Furthermore, three lithium ion cells, referred to as Test Group 3 wereconstructed with an anode comprising an anode material of the presentapplication comprising reduded graphene oxide encapsulated activematerial particles formed by the first and second active materialparticle modification processes (FIGS. 3A and 3B). Thus, the activematerial particles utilized in the Test Group 3 samples comprised a coreof crystalline silicon and a buffer layer of lithium silicate. It isnoted that the active material particles of the Test Group 3 samples hadan average particle size of about 400 nm.

In addition, lithium ion control cells were constructed with an anodehaving an electrochemically active material comprising silicon particlesencapsulated within crumpled graphene and having average particle sizesof about 50 nm and about 400 nm, respectively. In addition, the siliconparticles encapsulated within the crumpled graphene of theelectrochemically active material utilized in the control cells were notprocessed using either of the first or the second silicon processingsteps. Three lithium-ion control cells, referred to as Control Group 1,were constructed comprising an anode having an electrochemically activematerial that comprised silicon with a 50 nm particle size. Threelithium-ion control cells, referred to as Control Group 2, wereconstructed comprising an anode with an electrode active material thatcomprised silicon with a 400 nm particle size. Table I, shown below,identifies the compositions of the silicon particles utilized in therespective anode active materials of the test and control group cells.

TABLE I Average Silicon Silicon Silicon Buffer Cell Group Particle SizeTreatment layer Composition Test Group 1 50 nm 1 Silicon Oxide TestGroup 2 400 nm 1 Silicon Oxide Test Group 3 400 nm 1 and 2 LithiumSilicate Control Group 1 50 nm N/A N/A Control Group 2 400 nm N/A N/A

All test and control cells were subjected to a pulse discharge regimento test the specific capacity of the respective cells. Each of the cellswas tested at a 10C discharge rate to a predetermined threshold voltageof about 1.5 V. The pulse discharge regimen included a series ofalternating 5 second current pulse and 5 second rest periods until anominal voltage of 1.5V was reached.

FIGS. 5 and 6 illustrate the results of the pulse discharge testing. Asshown, the graphs show percent capacity retention as a function of thenumber of charge/discharge cycles. FIG. 5 illustrates the pulsedischarge testing results for the Test Group 1 cells in comparison tothe Control Group 1 cells. As shown, the Test Group 1 cells exhibited agreater capacity retention in comparison to the Control Group 1 cells.Specifically, as shown, the Test Group 1 cells exhibited a capacityretention percentage of about 77 percent after 100 charge/dischargecycles whereas the Control Group 1 cells exhibited a capacity retentionpercentage of about 72 percent after 100 charge/discharge cycles. Thesetest results appear to indicate that the buffer layer comprising siliconoxide enhanced the capacity retention in comparison to lithium cellsconstructed with anodes composed of the anode material that did notcomprise active material particles comprising a silicon core with asilicon oxide buffer layer.

FIG. 6 illustrates the pulse discharge testing results for the TestGroup 2 cells in comparison to the Control Group 2 cells. As shown, theTest Group 2 cells exhibited a greater capacity retention in comparisonto the Control Group 2 cells. As shown, after about 57 charge/dischargecycles, the Test Group 2 cells exhibited a capacity retention of about88 percent whereas the Control group 2 cells exhibited a capacityretention of about 83 percent. In addition, FIG. 6 illustrates the pulsedischarge testing results for the Test Group 3 cells in comparison tothe Control Group 2 cells. As shown, the Test Group 3 cells exhibitedgreater capacity retention in comparison to the Control Group 2 cells.As shown, after about 95 charge-discharge cycles, the Test Group 3 cellsexhibited a capacity retention of about 82 percent, whereas the ControlGroup 2 cells exhibited a capacity retention of about 70 percent. Thesetest results appear to indicate that the anode material of the presentinvention that included active material particles having a silicon corewith a buffer layer comprising lithium silicate enhances the capacityretention in comparison to lithium cells constructed with anodescomprising active material particles that do not include a buffer layerof lithium silicate.

TABLE II No. of Percent Disconnection Percent Cycles Improve- Parameterat Improve- to 80% ment 50 Cycles ment Control Group 1 71 — 0.141 — TestGroup 1 99 39 0.103 27 Control Group 2 67 — 0.151 — Test Group 2 95 420.116 23 Test Group 3 110 54 0.136 10

Table II, shown above, further illustrates the improvements in capacityretention that were exhibited by the Test Group cells in comparison tothe Control Group cells. As shown in Table II, the Test Group 1 cellsexhibited a 39 percent improvement in capacity retention over theControl Group 1 cells. As illustrated in the Table above, the Test Group1 cells reached an 80 percent capacity retention at the 99^(th)charge/discharge cycle whereas the Control Group 1 cells reached an 80percent capacity retention at charge/discharge cycle 71. In addition,the cells of Test Groups 2 and 3 also exhibited an increase in capacityretention in comparison to the Control Group 2 cells. Specifically, asshown in Table II, Test Group 2 cells exhibited a 42 percent increase incapacity retention in comparison to the Control Group 2 cells.Furthermore, the Test Group 3 cells exhibited an increase in capacityretention of about 54 percent as the Test Group 2 cells reached an 80percent capacity retention at charge/discharge cycle 110 in comparisonto cycle number 67 for the Control Group 2 cells.

These test results appear to indicate that incorporating active materialparticles comprising a crystalline silicon core and a buffer layercomprising silicon oxide, lithium silicate, and/or carbon, incorporatedwithin a crumpled capsule comprising graphene significantly improvescapacity retention. In particular, the Test Group 3 cells, whichcomprised an electrode composed of the electrochemically active materialcomprising particles of the active material comonent composed of asilicon core with an buffer layer of lithium silicate, exhibited anincreased capacity retention in comparison to Test Group 2 cells. TheTest Grop 2 cells comprised an electrode composed of theelectrochemically active material comprising active material particlescomprising a silicon core with an buffer layer of silicon oxide. It isnoted that the cells of Test Group 2 exhibited a greater improvement incapacity retention over the respective control group cells as comparedto the Test Group 1 cells. These test results appear to indicate thatutilizing silicon having a larger particle size as well as utilizingsilicon particles having a buffer layer comprising lithium silicateincrease capacity retention.

In addition to measuring capacity retention, a “disconnection parameter”value was also calculated from the test results. The disconnectionparameter is a unitless value that measures the relative cumulativecapacity loss from the electrical disconnection of the silicon withinthe material structure. Thus, the lower the value of the disconnectionparameter indicates less capacity loss that results from an electricaldisconnection of the silicon within the material network.

As shown in Table II, all test group cells exhibited a lower disconnectparameter value in comparison to the control group cells. This wouldappear to indicate that incorporating silicon particles having a bufferlayer comprising at least one of silicon oxide and lithium silicatewithin the electrochemically active material utilized in an anode oflithium-ion cells exhibits a lower “disconnection parameter” value andthus results in less capacity loss. In addition, it is noted that TestGroup 1 cells exhibited a lower disconnection parameter in comparison toTest Group 2 cells. This would indicate that in addition to utilizingsilicon particles having a buffer layer of silicon oxide within theelectrochemically active material, silicon particles having a smallerparticle size, i.e., a particle size of about 50 nm, exhibits a lowerdisconnection parameter and thus results in less capacity loss.

Thus, the results of the pulse discharge regimen clearly show thesignificance of the electrochemically active material particle structureand the optimized structure of silicon particles therein. It isappreciated that various modifications to the inventive conceptsdescribed herein may be apparent to those of ordinary skill in the artwithout departing from the spirit and scope of the present applicationas defined by the appended claims.

What is claimed is:
 1. A composite anode material comprising: capsulescomprising graphene, reduced graphene oxide, graphene oxide, or acombination thereof; and active material particles disposed inside ofthe capsules, each particle comprising: a core; and a buffer layersurrounding the core, wherein the buffer layer and the core comprisedifferent materials.
 2. The anode material of claim 1, wherein the corecomprises crystalline silicon.
 3. The anode material of claim 2, whereinthe buffer layer comprises Si₂O_(x), where 0<x<0.8.
 4. The anodematerial of claim 2, wherein the buffer layer comprises silicon oxide, alithium silicate, carbon, or a combination thereof.
 5. The anodematerial of claim 2, wherein the buffer layer comprises Li₂Si₂O₅,Li₂SiO₃, Li₄SiO₄, or a combination thereof.
 6. The anode material ofclaim 1, wherein: the capsules have an average particle size that rangesfrom about 0.5 μm to about 10 μm; and the particles have an averageparticle size that ranges from about 30 nm to about 500 nm.
 7. The anodematerial of claim 1, wherein the buffer layers have an average thicknessthat ranges from about 1 nm to about 50 nm.
 8. The anode material ofclaim 1, wherein the buffer layers have an average thickness that rangesfrom about 5 nm to about 10 nm.
 9. An anode, comprising: capsulescomprising graphene, reduced graphene oxide, graphene oxide, or acombination thereof; active material particles disposed inside of thecapsules, each particle comprising: a core; and a buffer layersurrounding the core, wherein the buffer layer and the core comprisedifferent materials; and a binder.
 10. The anode of claim 9, wherein:the core comprises crystalline silicon; and the buffer layer comprisesSi₂O_(x), where 0<x<0.8.
 11. The anode of claim 9, wherein: the corecomprises crystalline silicon; and the buffer layer comprises Li₂Si₂O₅,Li₂SiO₃, Li₄SiO₄, or a combination thereof.
 12. The anode of claim 9,wherein: the core comprises crystalline silicon; and the buffer layercomprises a combination of a silicon oxide, lithium silicates, andcarbon.
 13. The anode of claim 9, wherein: the binder comprisespolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),poly(acrylic) acid, polyethylenetetrafluoroethylene (ETFE), polyamides,and polyimides, polyethylene (UHMW), styrene-butadiene rubber,cellulose, polyacrylate rubber, or a mixture thereof; and the electrodehas an internal porosity that ranges from about 40 percent to about 60percent, as measured by the equation:${{Internal}\mspace{14mu} {Porosity}\mspace{14mu} (\%)} = {1 - \left( \frac{{measured}\mspace{14mu} {density}}{{theoretical}\mspace{14mu} {density}} \right)}$where the measured density is calculated by dividing the mass of a driedelectrode by its volume, and the theoretical density is the density ofthe electrochemically active material that is 100 percent dense.
 14. Amethod of fabricating an anode, the method comprising: mixing the anodematerial of claim 1 with a solvent and a binder to form a slurry;coating a substrate with the slurry; and drying the coated substrate toform the anode.
 15. The method of claim 14, wherein: the core comprisescrystalline silicon; the coating comprises silicon oxide, a lithiumsilicate, carbon, or a combination thereof; the binder comprisespolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),poly(acrylic) acid, polyethylenetetrafluoroethylene (ETFE), polyamides,and polyimides, polyethylene (UHMW), styrene-butadiene rubber,cellulose, polyacrylate rubber, or a combination thereof; and thesolvent comprises water, ethanol, isopropyl alcohol, ethylene glycol,ethyl acetate, polar protiac solvents, polar aprotic solvents,N-methyl-2-pyrrolidone, or a combination thereof.
 16. A method offorming an anode active material, the method comprising: heatingcrystalline silicon particles in an inert atmosphere at a firsttemperature ranging from about 700° C. to about 900° C.; heating theparticles at the first temperature in an oxidizing atmosphere, for afirst time period, to form oxidized particles that each comprise asilicon oxide layer surrounding a crystalline silicon core; and coolingthe oxidized particles in an inert atmosphere.
 17. The method of claim16, wherein: the first time period ranges from about 10 minutes to about40 minutes; and the particles are heated in a tube furnace.
 18. Themethod of claim 16, further comprising: mixing the oxidized particles, alithium salt, and a solvent to form a mixture; drying the mixture; andheating the dried mixture at a temperature ranging from about 600° C. toabout 700° C. in an inert atmosphere for a second time period, to formactive material particles that each comprise a buffer layer comprising alithium silicate surrounding a crystalline silicon core.
 19. The methodof claim 18, wherein: the lithium salt comprises lithium acetatedehydrate, LiO₂, LiS, LiPF₆, bis(oxalato)borate (LiBOB),oxalyldifluoroborate (LiODFB, fluoroalkylphosphate (LiFAP), or acombination thereof; the mixture comprises a weight ratio of theoxidized particles to the lithium salt ranging from about 1:0.5 to about1:0.75; and the mixture comprises a solids content ranging from about 5wt % to about 15 wt %, based on the total weight of the mixture.
 20. Themethod of claim 18, wherein: the lithium salt comprises lithium acetatedehydrate; and the buffer layer further comprises carbon.
 21. The methodof claim 18, further comprising using capillary compression toencapsulate the active material particles in capsules comprisinggraphene, reduced graphene oxide, graphene oxide, or a combinationthereof.